where \(z_\delta \)  is the \((1-\delta )\)  quantile of the standard normal
distribution, and \(IF\)  is an inflation factor to account for the loss
of power that results due to repeated testing relative to doing a
single final analysis. For example, the inflation factor associated
with using O'Brien-Fleming stopping boundaries[1]} is modest,
equal to about 1.03; see Tsiatis[2]}. Information at an
interim analysis at time \(t\)  is approximated as
\(\mbox{Inf}(t) = \big [ SE\lbrace  \widehat{\beta }(t)\rbrace \big ]^{-2}.\) 
